tags: #publish links: [[Gemstone Faceting]] created: 2023-01-01 Sun --- # GIA Colour grading system http://gemologyproject.com/wiki/index.php?title=Color_grading https://www.gemsociety.org/article/international-gem-society-cheat-sheets-for-gemologists/ https://www.gemsociety.org/article/evaluating-color-hue-tone-and-saturation/ Very roughly speaking: Format is: `[<modifiers>] <colour-abbreviation>, <tone>/<saturation>` Specifically: `[sl|st|vsl|vst] P|VP|V|BV|B|BG|G|YG|Y|YO|O|RO|R|R|RP|Pk|Brn, [2-8]/[1-6]` ## Colour On a colourwheel, using **P**urple-**V**iolet-**B**lue-**G**reen-**Y**ellow-**O**range-**R**ed Special cases: - **Pk** = Pink - **Brn** = Brown In-between colours are BG, YG etc (in either order) In-between in-betweens use **sl**ightly and **st**rongly and **v**ery modifiers and lowercasing, like this: - B - vslgB - vstgB - GB - vstbG - bG - vslbG - G Seemingly not all combinations are used: supposedly there are 31 combos in use - see the Gemology Project link above. In particular it is unclear when/whether the **v**ery modifier should be used. I suspect it's historical-names stuff. The actual grading system is GIA proprietary and you're supposed to pay for a course or something. Oddly enough that limits adoption, as do commercial concerns (e.g. people want their stones to fall in a valuable category, hence bicker over the category boundaries). ## Tone Brightness / "amount of white or black mixed in to the colour" (*not* saturation/vividness - that is **Saturation**) In theory this runs from 0 (transparent / clear) to 10 (opaque black) In practise 0 would not need a tone rating as a coloured gem, and black would not be a useful gemstone, so only 2-8 are actually used. ## Saturation Grey/brownness, purity/vividness of the colour (*not* brightness - that is **Tone**) From 1-6, 1=brown/gray, 6=vivid